关于对HD-10的一些问题
原文如下：
JVC introduces the JY-HD10 HD, the world’s first low-cost HD camcorder.
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JVC has announced the JY-HD10, the pro version of the consumer GR-HD1, which will be available in May and cost less than $4,000.
Is DV dead? That's the question being asked on video-oriented listservs since JVC demonstrated its GR-HD1 consumer HD camcorder at the Combined Exhibition of Advanced Technology (CEATEC) show in October. JVC's professional products division has announced the JY-HD10, its version of the consumer GR-HD1. The JY-HD10 includes a dual XLR adapter (integrated into the handle) with a mount for a shotgun mic. It will sell for less than $4,000 and be available in May.
The approximately 3lb. camcorder has a high-quality, all glass, 10X (f = 5.2mm-52mm) zoom lens with an optical image stabilizer. When shooting HD, the effective 35mm range is 40.3mm to 403mm. (GL-V0752 0.7X and GL-V1452 1.4X lens adapters are available.) The f1.8 to f1.9 lens has a 52mm filter mount. The JY-HD10 employs a single 1/3in., 1.18-megapixel CCD capable of generating both interlace and progressive images.
The JY-HD10 supports three recording modes: 4:3/true-16:9 NTSC DV; 16:9 SDTV (480p60) MPEG-2; and 16:9 HDTV (720p30) MPEG-2. When in DV mode, both 32kHz 12-bit and 48kHz 16-bit PCM recording are used. In the MPEG-2 modes, stereo audio is recorded at a data rate of 384kbps using MPEG-1, Layer-2, 48kHz, with 16-bit sampling. Audio levels are visible on the LCD monitor. In all modes, the signal is recorded to standard miniDV tape.
The 0.44in. color LCD viewfinder has 113,300 pixels, while the flip-out 3.5in. polycrystalline silicon LCD has 200,000 pixels. The camera's grip rotates 90 degrees for low-angle shooting. JVC claims a battery life of slightly more than one hour when shooting HD.
JPEG compressed still pictures can be recorded to an SD memory card. Four image capture resolutions are supported: 620×480, 848×480, 1280×720, and 1280×960. Up to 23 pictures at 1280x960 can be recorded to an 8MB card.
A YPbPr cable supplies analog component output for all recording formats. You can choose to output 480i/480p/720p (720p29.97 is upconverted to 720p59.94); 480p59.94 and 720p29.97 upconverted to 1080i; or downconvert all to 480i. Downconverted video is output on the camcorder's composite and S-Video jacks, and downconverted video can be letterboxed for display on 4:3 monitors.
An i.LINK connector inputs and outputs either DV25 or MPEG-2-TS (transport stream) data. Unlike when using analog component output, 720p is output as 720p29.97 video. A headphone jack and a microphone jack (not XLR) are available on the camcorder.
The camcorder has an “AV/S” connector (composite video, S-Video input/output, stereo audio input/output, and edit control) plus connectors for external DC power and USB. Also included with the camcorder: a CD-ROM with application software, shoulder strap, lens cap, editing cable, USB cable, a YPbPr component analog output cable, and an A/V cable. Neither SD nor HD can be recorded from an analog source.
Although the camcorder has a built-in edit controller, it functions only for DV recordings. Thus, it will not be possible to assemble-edit from the JY-HD10 to a JVC D-VHS deck. However, you can clone MPEG-2-TS segments to and from D-VHS.
Of course, while JVC is the first to announce a consumer HD camcorder, I'm certain we'll see announcements from other manufacturers during the year. This story is about more than a single product. JVC's announcement has raised important questions for those buying DV equipment. Few of us can forget how quickly the DV format, with FireWire, penetrated the intended prosumer market, as well as the industrial/broadcast market.
I've seen the same four questions repeatedly asked on the Internet: Can a camcorder that uses miniDV tape record “true” HD? Can a single-chip camcorder provide quality high enough to justify HD recording? How is it possible that an HD camcorder can be sold for approximately the price of a DV camcorder? How can an MPEG-2 recording be edited?
Since the first question must be answered before the other three have any relevance, let's start there. To answer it, let's look at a portion of the ATSC table that defines what's HDTV in the United States.
Above, text in green provides information about the two HDTV standards used for broadcasting. The critical parameters defining these standards are their aspect ratios (16:9) and their resolutions — 720 lines and 1080 lines. If a signal has these parameters and has a frame-rate of 24fps, 30fps, or 60fps, it is HDTV. And clearly, the 720p30 signal recorded by the JVC camcorder (indicated in blue) meets the ATSC requirements for HDTV.
The 720p30 temporal rate is half that of 720p60. While many will reject video with such low temporal resolution, others will embrace it as providing an inherent film look. JVC has not announced a European version that offers 720p25.
The FCC limits DTV (8VSB) bandwidth to 6MHz, which allows a maximum data rate of 19.4Mbps. Prior to broadcast, 1080i video undergoes a bit-rate reduction of 70:1 to about 18Mbps. Bit-rate reduction is primarily accomplished by MPEG-2 compression. The 720p60 image data rate is slightly less, at 16Mbps. To these image data rates, the AC-3 (Dolby Digital) data rate of 384kbps is added to generate the “program” data rate.
Note that compression ratios can be increased or decreased based upon application requirements. For example, 720p60 broadcast program data rates can range from 18Mbps (equal to 1080i) to 15Mbps (allowing the multicast of a 3Mbps SDTV subchannel). Pre-recorded D-VHS-HD D-Theater tapes from movie studios have a data rate of up to 28.2Mbps. They reportedly offer picture quality nearly equal that of uncompressed D-5 HD recordings of feature films.
All these data rates are easily recorded to miniDV tape. Such low data rates are possible because DV uses intra-frame compression, while MPEG-2 employs inter-frame compression.
Inter-frame compression is efficient because of its use of P and B frames in addition to I frames. P frames are based upon the previous I frame or P frame, while B frames are based on the previous and following P or I frame. As I frames are spaced farther apart and B frames — and to a lesser extent P frames — are placed between I frames, image quality remains nearly constant while the data rate is reduced. Typically, I frames occur every 15 frames, as shown in this sequence: IBBPBBPBBPBBPBB. The number of frames until the next I frame is the GOP (group of pictures) length.
To facilitate the editing process, a short, “closed” GOP structure can be employed. For the JY-HD10, JVC chose to use a six-frame GOP — IBBPBB. With a 15-frame GOP, there are two large I frames for every second of video. But with a six-frame GOP, there are five large I frames per second. A short GOP, plus the use of CBR rather than VBR encoding, requires a significant increase in the data rate to maintain image quality.
An Inexpensive High-Def CCD?
The table to the left shows the number of CCD elements required for 720-line and 1080-line HD for both 16:9 and 4:3 aspect ratio CCDs.
With a 16:9 chip, a DV recording is obtained by switching between two windows: a 480×720, 4:3 window and a 480×720, 16:9 window. Each window is centered vertically and horizontally on the CCD. The entire chip is used to obtain a 720p or 1080i image. Using a 4:3 chip requires switching amongst three windows. A 4:3, 480×720 window and a 16:9, 480×720 window are used for DV recording. These windows are centered vertically and horizontally on the CCD. A third 16:9 window is used to obtain either a 720p or a 1080i image. Unlike with a 16:9 chip, while this window uses the entire chip's width, it does not fill the height of the CCD.
Because 4:3 chips are in mass production, they are the natural choice from a cost basis. The lowest-resolution CCD that can provide a 4:3/16:9 NTSC DV image, a 16:9 SDTV image, and a 16:9 HDTV image is a 1.23 megapixel chip. This resolution is easily available today.
Sony's DCR-IP220 MicroMV camcorder has a 2.1-megapixel CCD, while JVC's GR-DV3000U DV camcorder uses a 1.33-megapixel CCD. The JY-HD10 employs a 1.18-megapixel CCD (effective 1.14 megapixels) that provides 840,000 pixels, rather than the 921,600-pixel image normally used for 720p. Thus, a CCD row has only about 1,166 elements, which is reasonable given that the necessary (for a single-CCD camera) optical and electrical antialiasing filters limit effective horizontal resolution.
While pro videographers may find the use of a single CCD worrisome, those who own a digital camera have few such concerns because we know they deliver excellent color quality. Nevertheless, it is true that while a single CCD delivers two luminance samples for each cluster of four filtered CCD elements, the chip is able to provide only a single red, green, and blue sample from the cluster. The JVC CCD is a new design that uses white, green, cyan, and yellow filters to deliver maximum effective vertical resolution.
The 2:1 ratio of luma samples to chroma samples is not that significant because both DV and MPEG-2 compression use color subsampling that reduces chroma resolution even further. NTSC DV compression uses 4:1:1 sampling, while HD MPEG-2 compression uses 4:2:0 sampling.
What's Inside a DV Camcorder?
The bulk of any camcorder includes the CCD(s), zoom lens, the camera control microprocessor, analog I/O circuitry, i.LINK interface, microphone, audio DSP, and the recording mechanism. The significant new components for an HD camcorder include: a better lens, optical stabilization (necessary to avoid the even higher resolution CCD that would be needed for electronic stabilization); a high-resolution viewfinder; and an MPEG-2 codec.
The image DSP must, of course, be enhanced to deal with a greater-than-NTSC frame rate (480p60 mode) and a much larger image (720p30 mode). And here, I believe, lies one reason for limiting the JY-HD10's frame rate to 30fps. An NTSC DSP chip must handle 10.4 megapixels per second, while a DSP chip for 720p60 must process 55.3 megapixels per second. By limiting the HD frame rate to 30fps, only 27.7 megapixels per second must be processed. Thus, the JY-HD10's 10-bit video DSP is not too costly for a consumer camcorder.
Lowering the frame rate to 30fps reduces the data rate flowing into the MPEG-2 encoder by 50%, which could allow an image-recording data rate of only 8Mbps. However, because of the use of a six-frame GOP, JVC set the image data rate to 18.6Mbps to provide maximum image quality. Therefore, the total program data rate recorded to tape is 19Mbps.
Can We Edit?
MPEG-2 has a reputation as a very difficult format to edit. JVC's solution is state of the art. The JY-HD10 sends an MPEG-2-TS bitstream directly to a computer's i.LINK connector. JVC supplies four Windows XP applications:
The i.LINK I/O utility writes MPEG-2-TS data to disk files. It can also be used to write MPEG-2-TS productions to a JY-HD10 or any JVC D-VHS deck. Supported decks include the HM-DH30000 deck and also the new SR-VD400US. Both D-VHS and JY-HD10 tapes are structured as MPEG-2-TS data.
The audio utility converts Windows audio formats to and from MPEG-1 Audio Layer-2 audio files so that background music and sound effects can be added to videos.
MPEG Edit Studio Pro 1.0 LE was developed by the R&D labs of KDDI in Japan. The software provides frame-accurate nonlinear editing of SD and HD MPEG-2 files. Edit Studio can import captured MPEG-2-TS files and MPEG-1 Audio Layer-2 audio files. Both titles and simple effects can be added. According to JVC, video inserts and audio inserts can also be performed. Edit Studio Pro generates MPEG-2-TS files that can be opened by the i.LINK I/O utility.
Edit Studio also generates MPEG-2-PS (program stream) files for use by Image Mixer DVD. Edit Studio Pro requires a 1.3GHz Pentium 4 (2GHz or greater recommended). No Mac version of this software is planned.
Image Mixer DVD creates productions and burns them to DVDs. These DVDs can be played in any DVD player, but the intention is that they will be played on a DVD player with progressive output and displayed on a 16:9 progressive presentation device. Obviously, the creation of HD DVDs is not yet possible. When HD-DVD burners become available in the future, they will open the doors to low-cost HD production.
Of course, more professional production alternatives will be developed by third parties. Windows XP has an i.LINK DirectX driver for the HM-DH30000 D-VHS deck. Also, public domain software exists that will input and output an MPEG-2-TS bitstream via an i.LINK port. There's no reason to believe the JY-HD10 will not get similar software support, including applications for OS X.
If an MPEG-2-TS decoder and an MPEG-2 encoder were included on a PCI capture board with i.LINK input, it would be possible to realtime-convert source tapes to I-frame MPEG-2 during capture. These components could be supplied in a box with i.LINK in and out ports. Both options would merge perfectly with Apple's work with Panasonic to support DVCPRO 50 and DVCPRO HD (I frame, MPEG-2) in Final Cut Pro.
Those of us who worked with fragile Hi8 tapes often bumped our acquisition format to an editing format. The same can be done with prosumer HD. A converter, such as AJA Video's HD10A, would transcode the 720p60 or 1080i analog component signal output by the JY-HD10 to uncompressed HD SDI. Even better would be a transcoder that inputs 720p30 MPEG-2-TS via i.LINK, and then uncompresses and outputs the video as 720p60 or 1080i via HD SDI. In either case, the HD SDI signal can be recorded by a D-5, HDCAM, or DVCPRO HD deck — or captured directly into a computer.
So is 25Mbps DV dead? Personally, the answer is yes. Like others now used to watching primetime TV, movies, and sports in HD, I've found NTSC simply doesn't have the resolution necessary for viewing on a large screen. I'm looking forward to shooting widescreen in a format that will look great on a big screen. Clearly, JVC is betting that I'm not the only one who feels this way.
By supplying HD acquisition, editing, distribution, and presentation products, JVC is also positioned to provide a low-cost HD solution to markets such as medical education, marketing, museums, and aquariums. And I expect that prosumer HD will be adopted by the indie film community. I see the next digital revolution coming — fast.
TABLE 1
ATSC Table 3, defining HD standards ATSC Standards Interlace 29.97 FPS (1080i) Progressive 59.94 FPS (720p60) Progressive 29.97 FPS (720p30)
1080×1920 16:9 HD 1080×1920 2M pixels
FCC broadcast bandwidth is 19.4Mbps. Image data rate is limited to 18Mbps.
720×1280 16:9 HD 720×1280 922k pixels 720×1280 922k pixels
With the same bit-reduction ratio as 1080i, the image data rate is 16Mbps. With the same bit-reduction ratio as 1080i, the image data rate is 8Mbps.
Green = HDTV standards used for broadcasting
Blue = Signal recorded by JVC's JY-HD10
TABLE 2
Number of CCD elements required for an HD image 16:9 native CCD 4:3 native CCD
720p: 720×1280 720×1280 >> 1 million elements 1280×960 >> 1.23 million elements
1080i: 1080×1920 1080×1920 >> 2 million elements 1920×1440 >> 2.76 million elements